Simulation of Radionuclide Migration in Groundwater Away From an Underground Nuclear Test
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As temperatures cool and gas pressures dissipate, components of the cavity gasses begin to condense in an order determined by their relative vapor pressures or boiling points. First among these are condensing rock vapors that accumulate into a melt glass puddle at the bottom of the cavity. Groundwater eventually refills the cavity region. Radionuclides associated with an underground nuclear explosion are derived from the original materials in the device, nuclear reactions connected with the explosion, and activation products created in the geologic medium. Complex dynamic processes occurring milliseconds to hours after detonation will control their chemical nature and spatial distribution. Most radionuclide vapors will be retained in the immediate cavity region by rebounding compressive stresses in the formation. In some cases, small amounts of radionuclides may escape the cavity region as a result of pressure-driven "prompt injection". During cooling, heavier radionuclides with higher boiling points (such as 241Am or 2 39Pu) will condense first and largely be incorporated 3 within the melt glass [5]. Lighter radionuclides (such as tritium, H) tend to condense later within
a "radioactive" or "exchange" volume surrounding the cavity, typically within 2 to 5 cavity radii about the testing point [3]. Other radionuclides will partially condense both within the melt and the rubble zone. Some radionuclides (such as 85Kr) may exist only as noncondensible gases and move outside the immediate vicinity of the cavity/chimney system. Little is known about how radionuclides are distributed within melt glass or exchange volume rubble, nor of their chemical state in the rubble following condensation. Some may become associated with the solids of the chimney or cavity, while others, including the noncondensibles, may become incorporated within pore waters. When groundwater infills the cavity, the "rubble" fractions may form aqueous species or solid phases consistent with the aqueous chemistry and minerals in the rubble. THE CAMBRIC TEST The Cambric nuclear test was conducted at Frenchman Flat in NTS in 1965. Frenchman Flat, located in the southeast corner of the NTS, is an intermountain basin formed by Tertiaryage faulting typical of the Basin and Range physiographic province. The working point and resulting test cavity are centered in Quaternary/Tertiary alluvium, approximately 70 m beneath the ambient water table and 290 m beneath the ground surface. The alluvium is composed of interbedded silts, clays, sands and gravels derived largely from silicic volcanic rocks (tuff and rhyolitic lava). Alteration minerals include clinoptilolite, calcite, smectite, illite/muscovite, and iron oxide, all of which may possess sorptive potential. Site Data and Simulation Approach As reviewed in [6-8], the Cambric test had a small yield (0.75 kt) which produced cavity and exchange volumes approximately 10.9 m and 18 m in radius, respectively. The melt debris is 3 comprised of approximately 900 metric tons of glass that occupies a bulk region of 4
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